Semiconductor technology has improved dramatically in the last few decades. As a benchmark, the minimum feature size, i.e., the dimension of the smallest feature actually fabricated on a silicon wafer, has been reduced from several microns to about 90 nm and it is expected to continuously shrink with the development of nanotechnology. As a result, a highly integrated semiconductor die can host tens of millions of transistors. The side effect of such achievement is that a small manufacturing defect occurring to any individual transistor, e.g., an opening at its electrical contact, may escalate to a serious circuit-level quality issue.
As one of the most versatile instruments available for the examination and analysis of the micro-structural characteristics of solid objects, the scanning electron microscope (SEM) has been used extensively in the semiconductor industry to detect various types of manufacturing defects. The primary reason for choosing the SEM is its high resolution, which makes it possible to identify those defects that are visible only by examining the significantly magnified surface of a die. Currently, the typical resolution of a commercial SEM is 1.5 nm.
The basic principle of SEM is illustrated in conjunction with FIG. 1 in which a primary electron beam 100 bombards the surface 110 of a specimen 120. The trajectory of one beam electron 102 inside and outside the specimen 120 is highlighted for illustrative purposes. When a particle like the beam electron 102 impinges upon the surface 110 of the specimen 120, the interactions between the beam electron 102 and the particles that constitute the specimen can be divided into two categories: (1) elastic scattering, (2) and inelastic scattering, see FIGS. 1(B) and (C), respectively.
When elastic scattering occurs, the magnitude of the beam electron 130's velocity remains virtually constant. As a result, the kinetic energy,       E    =                  1        2            ⁢              m        e            ⁢              v        2              ,where me is the electron's mass and v is its velocity, is unchanged. In general, there is less than 1 eV of energy transferred from the beam electron 130 to a specimen particle 135. Such energy loss is negligible compared to the kinetic energy of the beam electron 130, which is typically at the level of several KeV or higher. However, the direction of the beam electron 130 is often different from its original direction by an angle φelastic, ranging from 0° up to 180°. This is because elastic scattering results from collisions of electrons with atomic nuclei, whose mass is significantly larger than that of the electrons.
In contrast, when inelastic scattering occurs, a significant amount of the kinetic energy is transferred from a beam electron 140 to the specimen particles 145. As a result, the kinetic energy of the beam electron 140 decreases after the collision. However, the direction change caused by inelastic scattering (represented by the angle φelastic) is usually far smaller than the direction change caused by elastic scattering.
Referring again to FIG. 1(A), when beam electron 102 enters specimen 120, it randomly collides with the specimen particles along its trajectory. There is a possibility that the beam electron 102 may be backscattered out of specimen 120 or be captured by a specimen particle and stay in specimen 120. Among the series of collisions, each inelastic scattering transfers a certain amount of a beam electron's kinetic energy to the particles, which sometimes leads to the ejection of loosely bound electrons from the particles. These ejected electrons are commonly referred to as secondary electrons, and the majority of them receive an initial kinetic energy of 0–50 eV. The secondary electrons along the trajectory of beam electron 102 can be grouped into two classes: (1) those secondary electrons 104 and 105 that are generated within a certain distance D to the specimen surface 110, and (2) those secondary electrons 106 that are generated inside the specimen beyond the distance D. The secondary electrons 104 and 105 in the first class may escape from the specimen surface 110 while the secondary electrons in the second class are very likely to be captured by other particles in the specimen.
Within the first class of secondary electrons, secondary electrons 104 are generated by the primary beam electron 102 while secondary electrons 105 are generated by the backscattered electron 108. The number of escaping secondary electrons 104 per unit area generated by the primary beam electron 102 is much greater than the number of secondary electrons 105 generated by the backscattered electron 108. Further, when the electron beam 100 scans across the specimen 120, the beam-produced secondary electrons 104 respond to local surface features of specimen 120 and therefore carry information about the specimen surface, while the secondary electrons 105 generated by the backscattered electrons do not include such information and they act more or less as background noise. Therefore, a secondary electron detector (not shown) is incorporated in a SEM to measure the magnitude of secondary electron current above the surface of the specimen 120 and the SEM generates an image of the specimen surface according to the current magnitude.
When a SEM scans a specimen using a primary electron beam, the primary beam electrons may stay in the specimen or be backscattered out of the specimen and the secondary electrons generated by the primary beam electrons may escape from the surface of the specimen or exit as part of a ground current if the specimen is grounded. If the number of electrons that enter the specimen and the number that leave the specimen are different, the specimen will be electrically charged and therefore have a nonzero voltage. If the net result is an increase of electrons in the specimen, the specimen will have a negative electrical potential which, in turn, repels primary beam electrons accessing the specimen surface. The specimen then appears darker in a SEM image due to a decrease of secondary electron current. In contrast, if the net result is a decrease of electrons in the specimen, the specimen will have a positive electrical potential that attracts more primary beam electrons hitting the specimen surface with higher electrical kinetic energy, producing a higher secondary electron current. Therefore, the specimen appears brighter in a SEM image.
When a die comprising different electrical components is exposed to a primary electron beam, different portions of the die may have different potentials depending upon their surface compositions. This phenomenon is also referred to as passive voltage contrast (PVC), since no external power supplies are involved. Accordingly, different portions of the die may have different brightness on the SEM image. Such brightness differences can be employed to detect certain defects such as electrical openings on the die. The brightness difference is directly related to the secondary electron current difference and therefore the electrical potential difference between the different portions on the die which, in turn, depends upon the dimension of those electrical components on the die. For example, the capacity of an electrically isolated conductor for accumulating electrons on its surface is a function of its surface dimension. However, the need to produce devices with ever smaller minimum feature size results in electrical components with ever smaller surface dimension. Thus, the brightness difference caused by PVC between different components is often too little to recognize on a conventional SEM image.
Therefore, it would be desirable to develop a method of identifying electrical failures in an integrated circuit on a die by maximizing the brightness difference between different components on a SEM image.